The PI's long-term goal is to understand genetic mechanisms that underlie leukemogenic transformation of the hematopoietic stem cell. Acute leukemia is one of the incurable, aggressive forms of malignancies with an estimated 28,000 new cases discovered each year and 21,000 deaths as a result of the disease. Of these the acute myelogenous leukemia (AML) is distinct because the malignant transformation is thought to occur in the myeloid stem cell, resulting in a clonal proliferation. Clinically, AML falls into two broad categories of good and bad prognosis. While good prognosis patients have no cytogenetic abnormalities or reciprocal translocations as sole anomalies, poor prognosis patients have multiple abnormalities and non-random deletions of chromosomes 5 and 7. Furthermore, AML arises either de novo (primary) or with a 12 fold increased risk (secondary) in patients treated with alkylating agents and radiotherapy for other cancers. Thus, the incidence of secondary myeloid neoplasms is likely to increase as we cure other forms of cancer and 50% of secondary myeloid malignancies harbor deletions of chromosome 5. As the regions of deletions are large, very little is known about critical genes from the deleted regions. The PI's laboratory has delineated that the band 5q13.3 is a target of deletion or disruption by translocation. They demonstrated that unbalanced translocations between chromosome 5 and other chromosomes result in loss or disruption of the 5q13.3 region. Thus genes with leukemia suppressor activity were postulated to reside within the 5q13.3 locus. Recently the Pl's laboratory has identified a novel gene SSBP2 (sequence specific single stranded DNA binding protein 2), homologous to a chicken gene encoding a protein that binds pyrimidine rich single stranded DNA sequences, as the target of disruption at 5ql3.3. Preliminary results suggest that transcripts from the remaining allele of SSBP2 are either undetectable or expressed at very low levels in primary leukemic blasts. More importantly, co-expression of SSBP2 inhibits activated ras (V-12H-ras) mediated transformation of NIH 3T3 fibroblasts. The studies proposed in this second-amended application will explore the mechanistic basis, nature and significance of SSBP2 mediated transformation inhibition. We will test the hypothesis that SSBP2 is a novel regulator of hematopoietic growth and differentiation and a loss or reduced expression of SSBP2 confers a proliferation/survival advantage to the leukemic progenitor. Three specific aims are proposed: 1) To conduct a comprehensive screen for leukemia specific somatic alterations in the SSBP2 gene and expression. 2) To determine cellular growth and myeloid differentiation pathways regulated by SSBP2 in order to understand the mechanism by which absence or decreased expression of SSBP2 promotes leukemogenesis. 3) To characterize the structural domains of SSBP2 required to mediate growth suppression. These studies may enhance our understanding of normal myeloid differentiation as well as the molecular basis of refractory MDS and AML and lead to improved diagnosis and therapy.